Aluminium substitution in iron(II-III)-layered double hydroxides:: Formation and cationic order

The formation and the modifications of the structural properties of an aluminium-substituted iron(II-III)-layered double hydroxide (LDH) of formula (Fe4Fe(2-6y)Al6yIII)-Fe-II-Al-III (OH)(12)SO4, 8H(2)O are followed by pH titration curves, Mossbauer spectroscopy and high-resolution X-ray powder diffraction using synchrotron radiation. Rietveld refinements allow to build a structural model for hydroxysulphate green rust, GR(SO42-), i.e. y = 0, in which a bilayer of sulphate anions points to the Fe3+ species. A cationic order is proposed to occur in both GR(SO42-) and aluminium-substituted hydroxysulphate green rust when y<0.08. Variation of the cell parameters and a sharp decrease in average crystal size and anisotropy are detected for an aluminium content as low as y = 0.01. The formation of Al-GR(SO42-) is preceded by the Successive precipitation of Fe-III. and Al-III (oxy)hydroxides. Adsorption of more soluble Al-III species onto the initially formed ferric oxyhydroxide may be responsible for this slowdown of crystal growth. Therefore, the insertion of low aluminium amount (y similar to 0.01) could be an interesting way for increasing the Surface reactivity of iron(II-III) LDH that maintains constant the quantity of the reactive Fe-II species of the material. (C) 2008 Elsevier Inc. All rights reserved.